Synaptic junctions are specializations of neuronal structure which support much of the intercellular communication which is essential to the function of the mammalian central nervous system. We have been using very high resolution electron microscopic methods to study components of synaptic junctions which support and modulate function at central synapses. The spatial resolution afforded by rapid freezing and freeze fracture techniques is sufficient that certain proteins can be identified simply by their appearance. We have learned to identify other proteins by comparing their appearance with that of purified proteins visualized by molecular shadowing, and are devising ways to apply immunocytochemical techniques in conjunction with freeze etching. Our studies of adult synaptic junctions have revealed details of the molecular organization in presynaptic axoplasm that contributes to presynaptic modulation of neurotransmitter output. We have detected heterogeneity in the protein architecture of the postsynaptic density which implies a much more dynamic role for this junctional component than previously appreciated. A complex cytoskeletal apparatus has been identified in dendritic spines which could be of fundamental importance in understanding the potential for plasticity and remodeling in the adult brain. We plan to continue studies of these junctional components at classes of synapse which permit additional functional correlations, and to extend the process of identifying the constituent proteins. Freeze fracture techniques have detected a difference in the postsynaptic structure of initial and mature junctions during synaptogenesis. We believe that these structural differences reflect the capacity of initial junctions to dissociate during the complex processes of synapse formation. We plan to extend these studies to include the presynaptic component of forming synapses, and have devised methods to examine membrane and cytoskeletal structure of growth cones, with particular attention to cell structure associated with static and relatively dynamic cell shape.